1. Field of Invention
This invention relates to electro-acoustic transducers and specifically to the type commonly referred to as compression drivers which are used in conjunction with acoustic horns, waveguides or directional baffles.
2. Brief Statement of the Prior Art
Compression drivers have traditionally been equipped with diaphragms having a spherical section radiating surface of area Ain, which conforms to a spherical input surface of a phasing/compression plug (acoustic transformer or equalizer). The acoustic pressure generated by movement of the diaphragm is directed into inlet apertures, in the form of slits or holes, on the spherical input surface of the compression plug through a plurality of passages that pass through the body of the compression plug to emerge from outlet ports which are collectively contained in a circular output region, called the throat of area Aout, on the front of the driver disposed towards the horn where Aout is less than Ain.
FIGS. 1 to 3 show various prior art compression plugs 250a, 250b, 250c used in conventional round throated compression drivers (not shown). As shown, the input apertures typically consist of distributed holes, concentric slits, radial slits and combinations thereof. The compression plug causes the air displaced by the diaphragm to be compressed and to emerge in planar phased coherence at the circular throat of the driver. FIG. 1 shows input apertures provided as concentric slits; FIG. 2 shows input apertures provided as radial slits; and FIG. 3 shows input apertures provided as distributed holes. In each figure, a dashed circle 251 represents the location of the compression driver""s round throat on the far side of the illustrated compression plugs.
Compression plugs for high frequency drivers have been designed with a chosen compression ratio, typically about 10:1, and with the distances between the inlet apertures being sufficiently small to enable a unique phase relationship up to the highest desired frequency which forms a plane wave at the circular throat on the front of the driver. This originated because the 1919 paper by A. G. Webster on the mathematical modeling of the acoustic characteristerics of horns with various flare equations was based on zero curvature assumptions. Thus, the predominant model of the day had generated a plane wave at the throat of the compression driver, which coupled to a acoustic horn, having a round input throat of equal diameter and in this model, the plane wave at the throat of the driver propagates through the horn and exits at the horn mouth, impossibly, as a non-divergent plane wave.
Acoustic horns and waveguides having non-circular throats with unequal height to width dimensions (non-unity aspect ratios), usually rectangular, are well known. As shown in FIG. 4, for example, multicell horns 200 generally have a rectangular throat 201 requiring that an intermediate acoustic coupler 210 that provides a round to square, or round to rectangular (unity to non-unity) transition from the circular throat of the compression driver (not shown) to the rectangular input throat 201 of the horn.
In attempts to avoid horizontal beaming of the acoustic output at the higher frequencies of the driver""s operating range, the horn""s rectangular input throat has evolved into a diffraction slot. As used herein, therefore, a diffraction slot is defined as an acoustically diffractive aperture with a non-unity aspect (height/width) ratio. The diffraction slot is typically, but not necessarily rectangular and according to this present specification, is necessarily of lesser area than that of the radiating diaphragm.
The objectives of this invention are to provide:
1) A large scale, high acoustic output, multi element, sectoral line array with coupled horizontal waveguide which, acoustically, radiates a wavefront at the mouth of the waveguide as would a ribbon radiator with a coupled waveguide; ie, having a straight isophase line; ie, having a cylindrical wavefront.
2) A compression driver and waveguide to satisfy the elemental requirements so that a cylindrical array of waveguide mouths collectively propagate sound energy so as to disobey the inverse square law by the closest approach to the theoretically attainable 3 dB between spherical and cylindrical radiation.
3) A compression driver with a slot throat which generates a concave isophase line along the major axis of the slot to propagate through the waveguide and emerge at its mouth straight.
4) Thus a phasing plug that results in a concave isophase line along the major axis of its output end, and straight or slightly convex across the diffracting minor axis.
5) A phasing plug of which the spherical input surface has apertures in the form of chordal slits in parallel array.
6) A compression driver which has a throat that is a slot.
7) compression drivers which may be directly coupled to an acoustic horn or waveguide having a diffraction slot at its throat.
8) Waveguides with a diffraction slot throat that requires no intermediate acoustic coupler for driver mounting, and no requirement for an internal diffraction slot in the waveguide.
9) High output, cylindrical radiator loudspeaker systems which are comprised of arrays of mouths of coupled waveguides and drivers in accordance with the above.
10) Large area, high output, plane radiator loudspeaker systems to most closely approach disobeyance of the inverse square law by the theoretically available 6 dB.
11) Arrayed loudspeaker systems projecting sound energy with maximum integrity, ie, minimum acoustic phase cancellations; loudest and clearest.
12) Arrayed loudspeaker systems whereby far field radiation conditions are approached at the mouth of each elemental waveguide and driver.
13) Arrayed loudspeaker systems with appropriate interface and control and signal processing for variable positioning of lobes.
Other and related objectives will be apparent from the following description of the invention.
This invention relates generally to a phasing/compression plug and the direct coupling of its acoustic output to a waveguide or horn having a slot throat. The plug has an input or primary end having a surface conforming to the contour of the radiating diaphragm and spaced therefrom and having a plurality of inlet apertures, preferably slits, in parallel array at spaced-apart increments, and it has a like plurality of output apertures in parallel and juxtaposed array on the secondary end of the plug body, which collectively form an output aperture within a region which has unequal length and width dimensions and which is of lesser area than the area of the input surface. A plurality of passages through the plug body connect each of the primary surface input apertures to a respective output aperture. The relative lengths of the passages are preselected to provide an acoustic wavefront which may be concave along its major (vertical) axis to achieve narrow vertical dispersion, and planar or convex across its minor (horizontal) axis to accomplish wide horizontal dispersion by diffraction.
The phasing/compression plug of the invention effects the transition of the bounds of the wavefront from round to a non-unity aspect ratio in a novel function of the plug such that the throat of the driver can be directly coupled to an acoustic waveguide or horn having a matching slot throat, thereby eliminating the requirement for a transition coupler and for a horn with an internal diffraction slot.
In a first aspect, the invention may be regarded as a phasing and compression plug for use in or with an electro-acoustic transducer, the plug comprising: a body with an input end having an input surface of area Ain and an output end having an output region of area Aout where Ain greater than Aout; a plurality of input apertures provided as chordal slits that are arranged in a substantially parallel, spaced-apart configuration on the input surface at the input end of the body; a corresponding plurality of output apertures contained in the output region at the output end of the body; and a plurality of passages through the body, each passage connecting one the plurality of input apertures with a corresponding output apertures, and expanding in area from the input apertures to the output apertures.
In a second aspect, the invention may be regarded as a phasing and compression plug for use in or with an electro-acoustic transducer, the plug comprising: a body with an input end having an input surface of area Ain and an output end having an output region of area Aout where Ain greater than Aout the output region having an non-unity aspect ratio; a plurality of input apertures on the input surface at the input end of the body: a corresponding plurality of output apertures contained in the output region at the output end of the body; a plurality of passages through the body, each passage connecting each of the plurality of input apertures with a corresponding output aperture, and expanding in area from the input apertures to the output apertures.
In a third aspect, the invention may be regarded as a phasing and compression plug for use in an electro-acoustic transducer having a diaphragm with a circular, contoured, vibrating surface, the plug having: an input end with an input surface of area Ain that conforms to the contour of said vibrating surface; an output end with a output region of area Aout where Ain greater than Aout, the output region having an non-unity aspect ratio with a major axis and a minor axis; a plurality of input apertures provided as chordal slits that are arranged in a substantially parallel, spaced-apart configuration on the input surface of said input end; a corresponding plurality of output apertures collectively contained in the output region at the output end of said plug; and a plurality of passages, one each extending from each of said input apertures on said input surface to a respective outlet aperture and expanding in area in the direction towards said outlet apertures.
In a fourth aspect, the invention may be regarded as a compression driver having a phasing and compression plug with a plurality of input apertures at an input end having an input surface of area Ain and with multiple passages leading to multiple output apertures at an output end and within an output region of non-unity aspect ratio and of area Aout where Ain greater than Aout, the compression driver having a throat continuing from the output region of the phasing and compression plug, and including means to mount said compression driver to a waveguide having a matching throat.